Magnetic recording medium and recording/reproducing method therefor

ABSTRACT

A magnetic recording medium comprising a ferromagnetic metal thin film layer primarily based on cobalt and formed on the surface of a flexible substrate, an organic topcoat layer containing a radiation-curable compound, an anti-oxidant, and optionally a lubricant on the surface of the metal thin film layer, and a backcoat layer containing an inorganic pigment, an organic binder, and a lubricant on the other surface of the substrate is provided with protrusions on its surface at an average density of at least 10 5  /a 2  per square millimeter of the surface, the protrusions having a height of 30 to 300 Å, where a is the distance of a gap in a magnetic head with which the medium is used in recording/reproducing operation.

This is a continuation of Ser. No. 022,536 filed on Mar. 4, 1987, nowabandoned which in turn is a continuation of Ser. No. 758,435 filed July24, 1985 and now abandoned.

CROSS REFERENCE TO RELATED APPLICATION

Reference is made to our copending application Ser. No. 749, 585, forMagnetic Recording Medium and Recording/Reproducing Method Therefor,filed June 27, 1985, and assigned to the same assignee as the presentinvention.

BACKGROUND OF THE INVENTION

This invention relates to magnetic recording media, and moreparticularly, to magnetic recording media of metal thin film type, and amethod for conducting recording/reproducing operation in such media.

Among magnetic recording media for use in video, audio and otherapplications, active research and development works have been made onmagnetic recording media, usually magnetic tapes having a magnetic layerin the form of a continuous thin film because of the compactness of aroll of tape.

The preferred magnetic layers for such continuous metal film type mediaare deposited films of Co, Co--Ni, Co--O, Co--Ni--O and similar systemsformed by the so-called oblique incidence evaporation process in whichcobalt and optional elements are evaporated in vacuum-and directed at agiven angle with respect to the normal to the substrate because suchevaporated films exhibit superior characteristics. These media shouldhave a flat surface because of remarkable deterioration of theirproperties due to a spacing loss. However, as the surface becomeflatter, the friction becomes greater adversely affecting head contactand transport movement.

Usually, the metal thin film type media have a magnetic layer as thin as0.05 to 0.5 μm so that the surface property of the media depends on thesurface property of the substrate. For example, Japanese PatentApplication Kokai No. 53-116115 discloses the provision of gentlysloping protrusions in the form of creases or wrinkles on the substratesurface. Also, Japanese Patent Application Kokai Nos. 58-68227 and58-100221 disclose the location of fine particles on the substratesurface, resulting in surface irregularities observable under an opticalmicroscope with a magnifying power of 50 to 400 and actually measurablefor height by means of a probe surface roughness meter. These media are,however, still insufficient in physical properties such as dynamicfriction, runnability (the durability of tape which travels infrictional contact with rigid members in a recording machine), andmoving stability as well as in electromagnetic properties.

Further, Japanese Patent Publication No. 39-25246 discloses theapplication of an organic lubricant on the surface of a ferromagneticmetal thin film layer as a topcoat layer for the purpose of reducingdynamic friction. The use of lubricant has the actually undesirableproblem that it tends to adhere to the associated head to eventuallyblock the head gap. At present, a technique for improving the surfaceproperties of a metal thin film type magnetic recording medium has notbeen established which can reduce dynamic friction and eliminate headadhesion accompanied by head gap blocking without interfering smoothmovement or adversely affecting electromagnetic properties.

SUMMARY OF THE INVENTION

It is, therefore, an object of the present invention to provide animproved metal thin film type magnetic recording medium which haseliminated head adhesion and head gap blocking without adverselyaffecting electromagnetic properties while exhibiting satisfactoryphysical properties including friction, runnability and movementstability.

It is another object of the present invention to provide a method forconducting recording/reproducing operation on such a magnetic recordingmedium.

A first aspect of the present invention is directed to a magneticrecording medium comprising a flexible substrate having opposed majorsurfaces, a ferromagnetic metal thin film layer on one surface of thesubstrate primarily comprising cobalt, an organic topcoat layer on thesurface of the metal thin film layer, and a backcoat layer on the othersurface of the substrate. The magnetic recording medium is to be used incombination with a magnetic head having a gap. The organic topcoat layercontains a radiation-curable compound, an anti-oxidant, and optionally alubricant. The backcoat layer contains an inorganic pigment, an organicbinder, and a lubricant. The metal thin film layer contains oxygen. Themedium has protrusions on its surface at an average density of at least10⁵ /a² per square millimeter of the surface, the protrusions having aheight of 30 to 300 Å, where a is the distance of the magnetic head gapas expressed in μm.

The present invention also provides a method for conductingrecording/reproducing operation on a magnetic recording mediumcomprising a flexible substrate having opposed major surfaces, aferromagnetic metal thin film layer on one surface of the substrateprincipally comprising cobalt, an organic topcoat layer on the surfaceof the metal thin film layer, and a backcoat layer on the other surfaceof the substrate, by passing the medium across a magnetic head having agap. The organic topcoat layer contains a radiation-curable compound, ananti-oxidant, and optionally a lubricant. The backcoat layer contains aninorganic pigment, an organic binder, and a lubricant. The metal thinfilm layer contains oxygen. The medium has in average at least 10⁵ /a²protrusions per square millimeter of the topcoat layer surface, theprotrusions having a height of 30 to 300 Å, where a is the distance ofthe magnetic head gap as expressed in μm.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, features, and advantages of the presentinvention will be more fully understood by reading the followingdescription when taken in conjunction with the accompanying drawings, inwhich:

FIG. 1 is a cross-sectional view of a magnetic recording mediumaccording to one embodiment of the present invention; and

FIG. 2 is an elevation of one example of a magnetic head used in themethod of the present invention.

It should be noted that the drawings are not drawn to scale and thecomponents are disproportionately depicted for purposes of illustration.

DETAILED DESCRIPTION OF THE INVENTION

Referring to FIG. 1, there is illustrated a magnetic recording mediumgenerally designated at 10 according to the present invention. Themagnetic recording medium 10 includes a substrate 11 having opposedmajor surfaces, a ferromagnetic metal thin film layer 12 formed on onemajor surface of substrate 11, a topcoat layer 13 on the surface ofmetal thin film layer 12, and a backcoat layer 14 formed on the othermajor surface of substrate 11. The medium 10 has randomly distributedprotrusions or bosses 16 on the surface, preferably on the topcoatsurface. These elements will be described in more detail hereinafter.

Substrate

The substrates on which the ferromagnetic metal thin film layer isformed are not particularly limited as long as they are non-magnetic.Particularly preferred are flexible substrates, especially, of resins,for example, polyesters such as polyethylene terephthalate andpolyimides.

The substrate may preferably have a thickness of 5 to 20 μm.

Protrusion

Fine protrusions or bosses 16 as shown in FIG. 1 have a height h of 30to 300 Å, and more particularly, 50 to 200 Å. The protrusions providedin the present invention have such dimensions that they are notobservable under an optical microscope or measurable by a probe typesurface roughness meter, but can only be observable under a scanningelectron microscope. Larger protrusions in excess of 300 Å which areobservable under an optical microscope are not desirable because ofdeterioration in electromagnetic properties and movement stability.Smaller protrusions of lower than 30 Å are not effective in improvingphysical properties.

The protrusions should be distributed on the surface of the magneticrecording medium at an average density of at least 10⁵ /a², and morepreferably 2×10⁵ /a² to 1×10⁹ /a² per square millimeter of the surface.A magnetic head 20 with which the magnetic recording medium of thepresent invention is used is provided with a gap 24 having a distance aas shown in FIG. 2. The gap distance a usually ranges from 0.1 μm to 0.5μm, and more preferably, from 0.1 μm to 0.4 μm. At protrusion densitiesof less than 10⁵ /a² /mm², and more particularly less than 2×10⁵ /a²/mm², there result increased noise, deteriorated still performance, andother disadvantages, which are undesirable in practical applications.Higher protrusion densities of more than 10⁹ /a² /mm² are rather lesseffective in improving physical properties because of deterioration inelectromagnetic properties.

The protrusions 16 may generally be provided by placing submicronparticles 15 on the surface of the substrate as clearly shown in FIG. 1.The submicron particles used herein have a particle size of 30 to 300 Å,and more preferably 50 to 200 Å. Submicron protrusions are then formedon the topcoat surface of the magnetic recording medium which conform tothe submicron particles on the substrate surface in shape and size.

The submicron particles used in the practice of the present inventionare those generally known as colloidal particles. Examples of theparticles which can be used herein include SiO₂ (colloidal silica), Al₂O₃ (alumina sol), MgO, TiO₂, ZnO, Fe₂ O₃, zirconia, CdO, NiO, CaWO₄,CaCO₃, BaCO₃, CoCO₃, BaTiO₃, Ti (titanium black), Au, Ag, Cu, Ni, Fe,various hydrosols, and resinous particles. Inorganic particles arepreferred among others.

The submicron particles may be placed on the substrate surface, forexample, by dispersing them in a suitable solvent to form a dispersion,and applying the dispersion to the substrate followed by drying. Anyaqueous emulsion containing a resinous component may also be added tothe particle dispersion before it is applied to the substrate. Theaddition of a resinous component allows gently-sloping protrusions toform in conformity-to the particles although it is not critical in thepresent invention.

Alternatively, the submicron particles may be contained in the topcoatlayer to provide protrusions rather than the placement of particles onthe substrate surface.

Magnetic layer

The magnetic recording medium of the present invention has a magneticlayer on a substrate. The magnetic layer is of continuous ferromagneticmetal thin film type coextending over the substrate and is generallybased on cobalt. In preferred embodiments of the present invention, themagnetic layer may preferably consist essentially of cobalt; cobalt andoxygen; cobalt, oxygen and nickel and/or chromium. That is, the magneticlayer may consist essentially of cobalt alone or a mixture of cobaltwith nickel and/or oxygen.

Where the layer consists essentially of cobalt and nickel, the weightratio of Co/Ni may preferably be at least about 1.5.

The magnetic layer may further contain oxygen in addition to cobalt orcobalt and nickel. The presence of oxygen contributes to furtherimprovements in electromagnetic characteristics and runnability. In thiscase, the atomic ratio of O/Co (when nickel free) or O/(Co+Ni) ispreferably not more than about 0.5, and more preferably from about 0.05to 0.5.

Better results are obtained when the ferromagnetic metal thin film layercontains chromium in addition to cobalt; cobalt and nickel; cobalt andoxygen; or cobalt, nickel, and oxygen. The presence of chromiumcontributes to further improvements in electromagnetic characteristics,output level, signal-to-noise (S/N) ratio, and film strength. In thiscase, the weight ratio of Cr/Co (when nickel free) or Cr/(Co+Ni) ispreferably in the range of about 0.001 to 0.1, and more preferably about0.005 to 0.05.

On the surface of the ferromagnetic metal thin film layer, oxygen formsoxides with ferromagnetic metals Co and Ni. In Auger spectroscopy, peaksindicative of oxides appear in a surface layer, particularly in asurface layer from the surface to a depth of 50 to 500 Å, morepreferably 50 to 200 Å. This oxide layer has an oxygen content of theorder of 0.5 to 1.0 in atomic ratio. No particular limit is imposed onthe concentration gradient of oxygen in the ferromagnetic metal thinfilm layer.

The ferromagnetic metal thin film layer may further contain traceelements, particularly transition metal elements, for example, Fe, Mn,V, Zr, Nb, Ta, Ti, Zn, Mo, W, Cu, etc.

The ferromagnetic metal thin film layer preferably consists of acoalescence of Co base particles of columnar structure oriented obliqueto the normal to the substrate. More specifically, the axis of particlesof columnar structure is preferably oriented at an angle of about 10 to70 degrees with respect to the normal to the major surface of thesubstrate. Each columnar particle generally extends throughout thethickness of the thin film layer and has a minor diameter of the orderof 50 to 500 angstroms. Cobalt and optional metals such as nickel andchromium form the columnar structure particles themselves while oxygen,when added, is generally present on the surface of each columnarstructure particle in the surface layer essentially in the form ofoxides. The ferromagnetic metal thin film layer generally has athickness of about 0.05 to 0.5 μm, and preferably about 0.07 to 0.3 μm.

The magnetic layer is generally formed by the so-called obliqueincidence evaporation process. The oblique incidence evaporation processmay be any of well-known techniques preferably using an electron beamgun while the minimum incident angle with respect to the normal to thesubstrate is preferably 30 degrees. Evaporation conditions andpost-treatments are well known in the art and any suitable ones may beselected therefrom. One effective post-treatment is a treatment forincorporating oxygen into the magnetic layer, which is also well knownin the art. For further information about this evaporation process,reference should be made to D. E. Speliotis et al., "Hard magnetic filmsof iron, cobalt and nickel", J. Appl. Phys., 36, 3,972 (1965) and Y.Maezawa et al., "Metal thin film video tape by vacuum deposition", IEREConference Proceedings 54 (The Fourth International Conference on Videoand Data Recording, The University of Southampton, Hampshire, England,Apr. 20-23, 1982), pp. 1-9.

The ferromagnetic metal thin film layer may be formed on the substrateeither directly or via an undercoat layer of the well-known type.Further, the ferromagnetic metal thin film lager is generally formed asa single layer, but in some cases, it may be made up from a plurality oflaminated sub-layers with or without an intermediate non-ferromagneticmetal thin film layer interposed therebetween.

The ferromagnetic metal thin film layer may be formed by any well-knowntechniques including evaporation, ion plating, and metallizing, and morepreferably by the so-called oblique incidence evaporation process. Theoblique incidence evaporation process may be any of well-knowntechniques preferably using an electron beam gun while the minimumincident angle with respect to the normal to the substrate is preferablyat least 20 degrees. Incident angles of less than 20 degrees result indeteriorated electromagnetic properties. The evaporation atmosphere maygenerally be an inner atmosphere of argon, helium or vacuum containingoxygen gas at a pressure of about 10⁻⁵ to 10⁰ Pa. Those skilled in theart will readily select other evaporation parameters includingsource-substrate spacing, substrate feed direction, can and maskconfigurations and arrangement, and the like, through a simpleexperiment if necessary.

Evaporation in an oxygen-containing atmosphere causes a metal oxide filmto form on the surface of the resulting magnetic layer. The partialpressure of oxygen gas necessary to allow for oxide formation may bereadily determined through a simple experiment.

A metal oxide coating may be formed on the surface of the magnetic layerby an oxidizing treatment. Any of the following oxidizing treatments maybe employed for this purpose.

(1) Dry treatment

(a) Energy particle treatment Oxygen may be directed as energy particlesto the magnetic layer at the final stage of evaporation process by meansof an ion gun or neutron gun as described in Japanese Patent ApplicationNo. 58-76640.

(b) Glow treatment

The magnetic layer is exposed to a plasma which is created by generatinga glow discharge in an atmosphere containing O₂, H₂ O or O₂ +H₂ O incombination with an inert gas such as Ar and N₂.

(c) Oxidizing gas

An oxidizing gas such as ozone and heated steam is blown to the magneticlayer.

(d) Heat treatment

Oxidation is effected by heating at temperatures of about 60° to 150° C.

(2) Wet Treatment

(a) Anodization

(b) Alkali treatment

(c) Acid treatment

Chromate treatment, permanganate treatment, phosphate treatment

(d) Oxidant treatment H₂ O₂

Topcoat

On the ferromagnetic metal thin film layer preferably having protrusionsis formed a specific organic topcoat layer according to the presentinvention. The topcoat layer formed in the practice of the presentinvention contains a radiation-curable compound, an anti-oxidant, andoptionally a lubricant.

The radiation-curable polymer, monomer and oligomer used in the topcoatlayer in the practice of the present invention may be thosethermoplastic resins having contained or incorporated in their moleculeradicals susceptible to crosslinking or polymerization upon exposure toradiation, for example, acrylic double bonds as given by acrylic andmethacrylic acids having an unsaturated double bond capable of radicalpolymerization or esters thereof; allyl double bonds as given by diallylphthalate; and unsaturated bonds as given by maleic acid and maleicderivatives.

Monomers which can be used as the polymer component in the practice ofthe present invention include acrylic acid, methacrylic acid, andacrylamide.

Those polymer having a double bond may be obtained by modifying variouspolyesters, polyols, polyurethanes and analogues with compounds havingan acrylic double bond. If desired, polyhydric alcohols or polyhydriccarboxylic acids may be blended to obtain compounds having varyingmolecular weights.

The foregoing examples are only a part of the radiation sensitive resinsused herein. They may also be used alone or in admixture.

The preferred organic binder in the topcoat layer is a compositioncomprising

(A) 20 to 70% by weight of a plastic compound having at least tworadiation-curable unsaturated double bonds and a molecular weight of5,000 to 100,000,

(B) 20 to 80% by weight of a rubber-like compound having at least oneradiation-curable unsaturated double bond or not being radiation curableand having a molecular weight of 3,000 to 100,000, and

(C) 10 to 40% by weight of a compound having at least oneradiation-curable unsaturated double bond and a molecular weight of 200to 3,000.

The components (A), (B), and (C) used in the backcoat binder accordingto the invention should contain per molecular at least 2, and preferablyat least 5 unsaturated double bonds for (A), at least 1, and preferablyat least 5 unsaturated double bonds for (B), and at least 1, andpreferably at least 3 unsaturated double bonds for (C).

The plastic compound used as (A) in the backcoat binder according to theinvention contains in its molecular chain at least two unsaturateddouble bonds capable of generating radicals upon exposure to radiationto produce a crosslinked structure. Such a compound may also be obtainedby modifying a thermoplastic resin so as to be radiation sensitive.

Illustrative radiation-curable resins are thermoplastic resins havingcontained or incorporated in their molecule groups capable ofcrosslinking or polymerizing upon exposure to radiation, for example,acrylic double bonds as given by acrylic and methacrylic acids having anunsaturated double bond capable of radical polymerization and estersthereof, allyl double bands as given by diallyl phthalate, andunsaturated bonds as given by maleic acid and maleic derivatives. Othercompounds having unsaturated double bonds capable of crosslinking orpolymerizing upon exposure to radiation may also be used as long as theyhave a molecular weight of 5,000 to 100,000, and preferably 10,000 to80,000.

Typical of the resins in the form of thermoplastic resins havingcontained in their molecule groups capable of crosslinking orpolymerizing upon exposure to radiation are unsaturated polyesterresins. Polyester resins having radiation-curable unsaturated doublebonds in their molecular chain are included, for example, unsaturatedpolyester resins which may be prepared by a standard process ofesterifying polybasic acids of (2) as described below and polyhydricalcohols into saturated polyester resins except that the polybasic acidsare partially replaced by maleic acid so that the resulting polyestersmay have radiation-curable unsaturated double bonds.

The radiation-curable unsaturated polyester resins may be prepared byadding maleic acid or fumaric acid to at least one polybasic acid and atleast one polyhydric alcohol, conducting water- or alcohol-removingreaction in a conventional manner, that is, in a nitrogen atmosphere at180° to 200° C. in the presence of a catalyst, raising the temperatureto 240° to 280° C., and conducting condensation reaction at thetemperature under a vacuum of 0.5 to 1 mmHg. The amount of maleic orfumaric acid added may be 1 to 40 mol %, and preferably 10 to 30 mol %of the acid reactant in consideration of crosslinking and radiationcurable properties during preparation.

Examples of the thermoplastic resins which can be modified intoradiation-curable resins will be described below.

(1) Vinyl chloride copolymers

Included are vinyl chloride-vinyl acetate-vinyl alcohol copolymers,vinyl chloride-vinyl alcohol copolymers, vinyl chloride-vinylalcohol-vinyl propionate copolymers, vinyl chloride-vinyl acetate-maleicacid copolymers, vinyl chloride-vinyl acetate-OH terminated, alkylbranched copolymers, for example, VROH, VYNC, VYBGX, VERR, VYES, VMCA,and VAGH (all trade names, manufactured by U.C.C.), and analogues. Thesecopolymers may be modified to be radiation sensitive by incorporatingacrylic, maleic, or allyl double bonds by a procedure as will bedescribed later.

(2) Saturated polyester resins

Included are saturated polyesters obtained by esterifying saturatedpolybasic acids such as phthalic acid, isophthalic acid, terephthalicacid, succinic acid, adipic acid, sebasic acid, etc. with polyhydricalcohols such as ethylene glycol, diethylene glycol, glycerine,trimethylolpropane, 1,2-propyleneglycol, 1,3-butanediol,dipropyleneglycol, 1,4-butanediol, 1,6-hexanediol, pentaerithritol,sorbitol, neopentylglycol, 1,4-cyclohexanedimethanol, etc., and productsobtained by modifying these resins with SO₃ Na or the like, for example,Vyron 53S (trade name, Toyobo K.K.). They may be modified to beradiation sensitive.

(3) Polyvinyl alcohol resins

Included are polyvinyl alcohol, butyral resins, acetal resins, formalresins, and copolymers of such units. They may be modified to beradiation sensitive by acting on the hydroxyl group in them by aprocedure as will be described later.

(4) Epoxy resins and phenoxy resins

Included are epoxy resins formed by reaction of bisphenol-A withepichlorohydrin and methyl epichlorohydrin, for example, Epicoat 152,154, 828, 1001, 1004, and 1007 (trade names, manufactured by ShellChemicals), DEN 431, DER 732, DER 511 and DER 331 (trade names,manufactured by Dow Chemicals), Epichlon 400 and 800 (trade names,manufactured by Dai-Nihon Ink K.K.); phenoxy resins which are epoxyresins having a high degree of polymerization, for example, PKHA, PKHC,and PKHH (trade names, manufactured by U.C.C.); and copolymers ofbrominated bisphenol-A with epichlorohidrin, for example, Epichlon 145,152, 153, and 1120 (trade names, manufactured by Dai-Nihon Ink K.K.).These resins may be modified to be radiation sensitive by using an epoxygroup contained therein.

(5) Cellulosic derivatives

A variety of cellulosic derivatives may be used although nitrocellulose,cellulose acetobutyrate, ethyl cellulose, butyl cellulose, acetylcellulose, and analogues are preferred. These resins may be modified tobe radiation sensitive by using a hydroxyl group contained therein.

Additional examples of the resins which can be subjected to radiationsensitive modification include polyfunctional polyester resins,polyether-ester resins, polyvinyl pyrrolidone resins and derivatives(e.g., PVP-olefin copolymers), polyamide resins, polyimide resins,phenol resins, spiro-acetal resins, hydroxyl-containing acrylic esters,and acrylic resins containing at least one methacrylate as a polymercomponent.

The high molecular weight compounds used as component (B) in the bindercomposition according to the invention are thermoplastic elastomers andprepolymers, and derivatives thereof modified to be radiation sensitive.The radiation sensitive modified compounds are more effective. Examplesof the elastomers and prepolymers are presented below.

(1) Polyurethane elastomers and prepolymers

Polyurethanes are very useful because of abrasion resistance andadhesion to substrates, for example, PET films.

Illustrative polyurethane elastomers and prepolymers are condensationproducts from (a) polyfunctional isocyanates such as2,4-toluenediisocyanate, 2,6-toluenediisocyanate,1,3-xylenediisocyanate, 1,4-xylenediisocyanate,1,5-naphthalenediisocyanate, m-phenylenediisocyanate,p-phenylenediisocianate, 3,3'-dimethyl-4,4'-diphenylmethanediisocyanate, 4,4'-diphenylmethane diisocyanate,3,3'-dimethylbiphenylene diisocyanate, 4,4'-biphenylene diisocyanate,hexamethylene diisocyanate, isophorone diisocyanate, dicyclohexylmethanediisocyanate, Desmodur L, Desmodur N (trade names, manufactured byFarbenfabriken Bayer A.G.), etc.; and (b) linear saturated polyesters asproduced through polycondensation from polyhydric alcohols (such asethylene glycol, diethylene glycol, glycerine, trimethylol propane,1,4-butanediol, 1,6-hexanediol, pentaerythritol, sorbitol,neopentylglycol, 1,4-cyclohexanedimethylol, etc.) and saturatedpolybasic acids (such as phthalic acid, isophthalic acid, terephthalicacid, succinic acid, adipic acid, sebasic acid, etc.); linear saturatedpolyethers such as polyethylene glycol, polypropylene glycol, andpolytetramethylene glycol; caprolactam; polyesters such ashydroxyl-containing acrylates and hydroxyl-containing methacrylates, andthe like. It is very useful to react the isocyanate or hydroxyl terminalgroup of these urethane elastomers with a monomer having an acrylic orallyl double bond to modify them to be radiation sensitive. Polymers mayalso contain hydroxy or carboxyl terminal groups as polar groups.

(2) Acrylonitrile-butadiene copolymerized elastomers

Acrylonitrile-butadiene copolymerized prepolymers having a hydroxylterminal group commercially available as Poly BD Liquid Resin fromSinclair Petro-Chemical and elastomers commercially available as Hiker1432J from Nihon Zeon K.K. are adequate because the double bond in thebutadiene unit is capable of generating a radical upon exposure toradiation to facilitate crosslinking and polymerization.

(3) Polybutadiene elastomer

Low molecular weight prepolymers having a hydroxyl terminal groupcommercially available as Poly BD Liquid Resin R-15 from SinclairPetro-Chemical and the like are preferred because they are compatiblewith thermoplastic resins. R-15 prepolymers whose molecule is terminatedwith a hydroxyl group can be more radiation sensitive by adding anacrylic unsaturated double bond to the molecule end, which is moreadvantageous as the binder component.

Also, cyclic products of polybutadienes commercially available asCBR-M901 from Nihon Synthetic Rubber K.K. offer satisfactory qualitywhen combined with thermoplastic resins.

Additional preferred examples of the thermoplastic elastomers andprepolymers include styrene-butadiene rubbers, chlorinated rubbers,acrylic rubbers, isoprene rubbers, and cyclic products thereof(commercially available as CIR 701 from Nihon Synthetic Rubber K.K.)while elastomers, for example, epoxy-modified rubbers and internallyplasticized, saturated linear polyesters (commercially available asVyron #300 from Toyobo K.K.) may also be useful provided that they aresubjected to radiation sensitive modification as will be describedlater.

Finally, component (C) or radiation-curable resins having unsaturateddouble bonds used in the topcoat composition according to the inventionwill be described.

Included are styrene, ethylacrylate, ethylene glycol diacrylate,ethylene glycol acrylate, diethylene glycol dimethacrylate,1,6-hexaneglycol diacrylate, 1,6-hexaneglycol dimethacrylate,trimethylolpropane triacrylate, trimethylolpropane trimethacrylate,polyfunctional oligoester acrylate (e.g., Aronix M-5400, 5500, 5700,7100 manufactured by Toa Synthetic K.K.), acryl-modified products ofurethane elastomers (e.g., Nippolan 4040), and products thereof having afunctional group like COOH incorporated therein.

Next, processes for the synthesis of the radiation curable binders willbe described.

(a) Synthesis of acryl-modified products (radiation sensitive modifiedresins) of vinyl chloride-vinyl acetate copolymeric resins.

A 5-liter four-necked flask is charged with 750 parts of a partiallysaponified vinyl chloride-vinyl acetate copolymer having a OH group(average polymerization degree n=500), 1250 parts of toluene, and 500parts of cyclohexanone. After the flask is heated at 80° C. to dissolvethe contents into a solution, 61.4 parts of 2-hydroxyethyl methacrylateadduct of tolylenediisocyanate (the preparation will be described later)is added and then, 0.012 parts of tin octylate and 0.012 parts ofhydroquinone are added. Reaction is continued in a N₂ stream at 80° C.until the reaction rate of NCO reaches 90%. At the end of reaction, thereaction solution is cooled and 1250 parts of methyl ethyl ketone isadded for dilution.

Preparation of 2-hydroxyethyl methacrylate (2HEMA) adduct oftolylenediisocyanate (TDI)

In a 1-liter four-necked flask, 1348 parts of TDI is heated at 80° C. ina N stream. A mixture of 260 parts of 2-ethylene methacrylate, 0.07parts of tin octylate, and 0.05 parts of hydroquinone is then addeddropwise while the reactor is cooled so as to control the temperature to80° to 85° C.

After the dropwise addition, the reaction is continued to completion at80° C. for 3 hours with stirring. At the end of reaction, the contentsare taken out of the flask and cooled, obtaining a white paste-likeproduct which is 2HEMA adduct of TDI based on the preparation method.

(b) Synthesis of acryl-modified products (radiation sensitive modifiedresins) of butyral resins

A 5-liter four-necked flask is charged with 100 parts of a butyral resin(BM-S, manufactured by Sekisui Chemicals K.K.), 191.2 parts of toluene,and 71.4 parts of cyclohexanone. After the flask is heated at 80° C. todissolve the contents into a solution, 7.4 parts of 2HEMA adduct of TDI(synthesized as above) is added and then, 0.015 parts of tin octylateand 0.015 parts of hydroquinone are added. Reaction is continued in a N₂stream at 80° C. until the reaction rate of NCO reaches or exceeds 90%.At the end of reaction, the reaction solution is cooled and an amount ofmethyl ethyl ketone is added for dilution.

(c) Synthesis of acryl-modified products (radiation sensitive modifiedresins) of saturated polyester resins

A four-necked flask is charged with 100 parts of a saturated polyesterresin (Vyron RV-200, manufactured by Toyobo K.K.), 116 parts of toluene,and 116 parts of methyl ethyl ketone. After the flask is heated at 80°C. to dissolve the contents into a solution, 3.55 parts of 2HEMA adductof TDI (synthesized as above) is added and the, 0.007 parts of tinoctylate and 0.007 parts of hydroquinone are added. Reaction iscontinued in a N stream at 80° C. until the reaction rate of NCO reachesor exceeds 90%.

(d-1) Synthesis of acryl-modified products (radiation sensitive modifiedresins) of epoxy resins

After 400 parts of an epoxy resin (Epicoat 1007, manufactured by ShellChemicals) is dissolved in 50 parts of toluene and 50 parts of methylethyl ketone by heating, 0.006 parts of N,N-dimethylbenzylamine and0.003 parts of hydroquinone are added. The temperature is raised to 80°C. and 69 parts of acrylic acid is added dropwise. Reaction is continueduntil the acid value is lowered to below 5.

(d-2) Synthesis of acryl-modified products (radiation sensitive modifiedresins) of phenoxy resins

A 3-liter four-necked flask is charged with 600 parts of a OHgroup-bearing phenoxy resin (PKHH manufactured by U.C.C., molecularweight 30,000) and 1,800 parts of methyl ethyl ketone. After the flaskis heated at 80° C. to dissolve the contents into a solution, 6.0 partsof 2HEMA adduct of TDI (synthesized as above) is added and then, 0.012parts of tin octylate and 0.012 parts of hydroquinone are added.Reaction is continued in a N₂ stream at 80° C. until the reaction rateof NCO reaches or exceeds 90%. The resultant modified phenoxy producthas a molecular weight of 35,000 and one double bond per molecule.

(e) Synthesis of acryl-modified products (radiation-curable modifiedresins) of urethane elastomers

A reactor is charged with 250 parts of a urethane prepolymer ofisocyanate-terminated diphenylmethane diisocyanate (MDI) type (Nippolan3119 manufactured by Nippon Polyurethane K.K.), 32.5 parts of 2HEMA,0.07 parts of hydroquinone, and 0.009 parts of tin octylate and heatedat 80° C. to dissolve the contents into a solution. While the reactor iscooled so as to control the temperature to 80° to 90° C., 43.5 parts ofTDI is added dropwise. At the end of addition, reaction is continued at80° C. until the reaction rate reaches or exceeds 95%.

(f) Synthesis of acryl-modified products (radiation-curable modifiedelastomers) of terminally urethane-modified polyether elastomers

A reactor is charged with 250 parts of a polyether (PTG-500 manufacturedby Nippon Polyurethane K.K.), 32.5 parts of 2HEMA, 0.007 parts ofhydroquinone, and 0.009 parts of tin octylate and heated at 80° C. todissolve the contents into a solution. While the reactor is cooled so asto control the temperature to 80° to 90° C., 43.5 parts of TDI is addeddropwise. At the end of addition, reaction is continued at 80° C. untilthe reaction rate reaches or exceeds 95%.

(g) Synthesis of acryl-modified products (radiation-curable modifiedelastomers) of polybutadiene elastomers

A reactor is charged with 250 parts of a low-molecular weight,hydroxyl-terminated polybutadiene (Poly BD Liquid Resin R-15,manufactured by Sinclair Petro-Chemical), 32.5 parts of 2HEMA, 0.007parts of hydroquinone, and 0.009 parts of tin octylate and heated at 80°C. to dissolve the contents into a solution. While the reactor is cooledso as to control the temperature to 80° to 90° C., 43.5 parts of TDI isadded dropwise. At the end of addition, reaction is continued at 80° C.until the reaction rate reaches or exceeds 95%.

Among known polymers, polymers of one type degrade while polymers ofanother type give rise to crosslinking between molecules upon exposureto radiation.

Included in the crosslinking type are polyethylene, polypropylene,polystyrene, polyacrylate, polyacrylamide, polyvinyl chloride,polyester, polyvinyl pyrrolidone rubber, polyvinyl alcohol, andpolyacrolein. Since these polymers of the crosslinking type give rise tocrosslinking reaction without any particular modification as previouslydescribed, they may also be used as the radiation-curable backcoat resinas well as the above-mentioned modified products. The polymers of thecrosslinking type can be cured within a short time even when used insolventless form. The use of polymers of this type is thus very usefulas the topcoat resin.

The polymers may have functional groups such as a hydroxy group derivedfrom alcohols, phenols and phosphates, a carboxyl group derived fromaromatic and aliphatic compounds, a sulfone group, an amino group, andan ammonium group. The use of compounds having functional groupsimproves the adhesion of the topcoat to the ferromagnetic thin layer.

Particularly preferred for the radiation curable resin composition usedin the topcoat according to the invention are combinations of

component (A) selected from partially saponified vinyl chloride-vinylacetate copolymers having carboxylic units incorporated therein, vinylchloride-vinyl acetate-vinyl alcohol-maleic acid copolymers, epoxyresins, and compounds prepared by reacting isocyanate-bearing compoundsresulting from reaction between phenoxy resins preferably havingcarboxylic units such as phthalic acid, isophthalic acid, terephthalicacid, adipic acid, and sebasic acid incorporated therein, andpolyisocyanate compounds with acrylic or methacrylic compounds having afunctional group capable of reacting with the isocyanate group,

component (B) selected from compounds prepared by reacting isocyanatecompounds or polyols (polyurethane elastomers preferably having hydroxyor carboxyl group incorporated therein) resulting from reaction betweenpolyols and isocyanates with acrylic or methacrylic compounds having areactive functional group,

component (C) selected from polyfunctional acrylate or methacrylatemonomers, oligoester acrylates, and low molecular weight compounds of(B).

These compounds may be used alone or in admixture of two or more.

The use of the radiation-curable compounds not only improves theadhesion of the topcoat layer to the magnetic layer, but also reinforcesthe topcoat layer, that is, increases the breakage strength and abrasionresistance of the topcoat layer, resulting in stable movement inhigh-temperature conditions. There is thus obtained a magnetic recordingmedium which is improved in dropout, head adhesion, topcoat abrasion,and friction variation with use. A roll of the magnetic recording mediumbeing taken up during manufacture is ho longer tightened during curingprocess so that the medium has uniform properties in a lengthwisedirection.

In the absence of the radiation-curable compounds in the topcoat layer,the movement of the resulting tape at elevated temperatures is oftenobstructed because the topcoat layer is severely abraded away to causedebris deposition on the head.

The use of radiation-curable binders allows the topcoat to becontinuously formed on a manufacturing line, contributing to energy andcost savings.

Any anti-oxidants may be used herein as long as they can preventoxidation of metals. The anti-oxidants used herein may be selected fromconventional anti-oxidants which may be generally classified into thefollowing groups:

(1) Phenolic anti-oxidants

(2) Amine anti-oxidants

(3) Phosphorous anti-oxidants

(4) Sulfur anti-oxidants

(5) Organic acid, alcohol and ester anti-oxidants

(6) Quinone anti-oxidants

(7) Inorganic acid and inorganic salt anti-oxidants.

Examples of each of these anti-oxidants are shown below.

(1) Phenolic anti-oxidants

2,6-di-tert-butyl-p-cresol,

2,6-di-tert-butylphenol,

2,4-dimethyl-6-tert-butylphenol,

butylhydroxyanisole,

2,2'-methylenebis(4-methyl-6-tert-butylphenol),

4,4'-butylidenebis(3-methyl-6-tert-butylphenol),

4,4'-thiobis(3-methyl-6-tert-butylphenol),

tetrakis[methylene-3-(3,5-di-tert-butyl-4hydroxyphenyl)propionate]methane,

1,1,3-tris(2-methyl-4-hydroxy-5-tert-butylphenyl)butane,

dibutylhydroxytoluene,

propyl gallate,

guaiacum resin,

nordihydroguaiaretic acid, etc.

Also included are phenolic anti-oxidants of radiation curable type, forexample, acrylate and methacrylate modified compounds of monoglycolsalicylate, 2,5-di-tert-butylhydroquinone, 2,4-dihydroxybenzophenone,2,4,5-trihydroxybutyrophenone, hydroquinone, etc.

(2) Amine anti-oxidants

phenyl-β-naphthylamine,

α-naphthylamine,

N,N'-di-sec-butyl-p-phenylenediamine,

phenothiazine,

N,N'-diphenyl-p-phenylenediamine,

alkanol amines,

phospholipid, etc.

Also included are amine anti-oxidants of radiation curable type, forexample, dimethylaminoethyl methacrylate and acrylate, and vinylderivatives.

(3) Phosphorous anti-oxidants

Included are phosphate esters of both radiation curable and radiationuncurable types. The R moiety of phosphates may include alkyl radicals,alkyl phenyl radicals, ethylene oxide, propylene oxide, etc. andpreferably contain 1 to 26 carbon atoms, and most preferably 1 to 22carbon atoms. The phosphates include mono-, di-, and tri-esters and theymay be used alone or in admixture. Mixtures comprising a majorproportion of mono- and di-esters are preferred and the tri-esters maybe excluded.

Also included in the phosphate esters are NH₄ type and methacrylate,acrylate, and vinyl modified types.

Illustrative examples include phosphites such as triphenyl phosphite,trioctadecyl phosphite, tridecyl phosphite, trilauryl trithiophosphite,etc.; hexamethyl phosphoric triamide, butyl phosphate, cetyl phosphate,butoxyethyl phosphate, 2-ethylhexyl phosphate, β-chloroethyl phosphate,butoxyethyl phosphate diethylamine salt, di(2-ethylhexyl) phosphate,ethyleneglycol acid phosphate; methacrylate and acrylate phosphates suchas 2-hydroxyethylmethacrylate phosphate, butylhydroxymethacrylatephosphate, caprylhydroxylmethacrylate phosphate,myristylhydroxymethacrylate phosphate, stearylhydroxymethacrylatephosphate, cetylhydroxymethacrylate phosphate,butylphenylhydroxymethacrylate phosphate, amylphenylhydroxymethacrylatephosphate, nonylphenylhydroxymethacrylate phosphate, and similaracrylate phosphates; phenyl phosphates such as phenyl phosphate andnonyl phosphate; alcoholic phosphates; vanadium series acidicphosphates, and the like.

The phosphate esters may be prepared by any well-known methods, forexample, as disclosed in Japanese Patent Publication No. 57-44223.

(4) Sulfur anti-oxidants

dilaurylthiodipropionate,

distearylthiodipropionate,

laurylstearylthiodipropionate,

dimyristylthiodipropionate,

distearyl-β,β'-thiobutyrate,

2-mercaptobenzoimidazole,

dilaurylsulfide., etc.

Also included are radiation curable methacrylate, acrylate modifiedcompounds of 4,4'-thio-bis(3-methyl-6-tert-butylphenol),2,2'-thio-bis(4-methyl-6-tert-bytylphenol), etc. They may furthercontain ethylene oxide and propylene oxide units.

(5) Organic acid, alcohol, and ester anti-oxidants

Included are sorbitol, glycerine, propylene glycol, adipic acid, citricacid, ascorbic acid, etc. as well as radiation curable derivativesthereof.

(6) Quinone anti-oxidants

Included are hydroquinone, tocopherol, etc. as well as radiation curablederivatives thereof.

(7) Inorganic acid and inorganic salt anti-oxidants

Phosphoric acid is a typical example.

In order to minimize the transfer of the topcoat substance to the backsurface of the magnetic recording medium in a roll form, radiationcurable anti-oxidants having an acrylic double bond in their moleculeare preferred, for example, monoglycol salicylate methacylate andacrylate, 4-tert-butylcatechol methacrylate and acrylate,dimethylaminoethyl methacrylate and acrylate, ethylhydroxymethacrylateand acrylate phosphates, cetylhydroxyphosphate methacrylate andacrylate, stearyl methacrylate and acrylate phosphates, and phenylderivatives of the foregoings,2,2'-thio-bis(4-methyl-6-tert-butylphenol) methacrylate and acrylate,etc.

The radiation curable anti-oxidants can be on-line cured to theferromagnetic thin film during manufacturing, eliminating thedeterioration of surface properties or output reduction due tosubsequent heat curing which makes a roll tighter to cause theconformity of the topcoat layer to the surface morphology of the backside.

The topcoat is formed on the ferromagnetic thin film to a thickness of10 to 100 Å, and more preferably to a thickness of 10 to 50 Å, as willbe described hereinafter. Thicker topcoats result in a loss ofelectromagnetic properties and can be abraded away during operation,resulting in clogging of head gaps. Too thin topcoats are not effective.

The use of the radiation curable anti-oxidants offers benefits inproperties including prevention of dropouts and reduction of the outputdifference between outer and inner coils in a roll form, as well as thebenefit of on-line production.

The lubricants used in the topcoat layer in the practice of the presentinvention may be conventional lubricants commonly used in prior artmagnetic recording media, for example, silicone oil, fluorine oil, fattyacids, fatty acid esters, paraffins, liquid paraffins, and surfaceactive agents, etc. Among others, preferred are fatty acids and/or fattyacid esters.

Examples of the fatty acids used herein include fatty acids having atleast 12 carbon atoms, more illustratively, RCOOH where R is an alkylhaving at least 11 carbon atoms, such as caprylic acid, capric acid,lauric acid, myristic acid, palmitic acid, stearic acid, behenic acid,oleic acid, elaidic acid, linolic acid, linoleic acid, stearolic acid,etc.

The fatty acid esters used herein may be those esters of monobasic fattyacids having 12 to 16 carbon atoms with monohydric alcohols having 3 to12 carbon atoms, and those esters of monobasic fatty acids having atleast 17 carbon atoms with monohydric alcohols, the esters having 21 to23 carbon atoms in total.

The silicone oils used herein may be fatty acid-modified silicones andpartially fluorine-modified silicones. The alcohols used herein may behigher alcohols. The fluorine compounds may be those obtained byelectrolytic substitution, telomerization, and oligomerization.

Also, lubricants of radiation curable type may be used and preferred.The use of the radiation curable lubricants prevents the conformity ofthe topcoat layer to the surface morphology of the back side of themedium in a roll form and thus offers benefits in properties includingprevention of dropouts and reduction of the output difference betweenouter and inner coils in a roll form, as well as the benefit of on-lineproduction.

The radiation curable lubricants are compounds having a chain moietycapable of providing lubricity and an acrylic double bond in theirmolecule, for example, acrylates, methacrylates, vinyl acetates,acrylamides, vinyl alcohol esters, methylvinyl alcohol esters,allylalcohol esters, glycerides, etc.

These lubricants may be represented by the following structuralformulas: ##STR1## where R is selected from straight chain and branched,saturated and unsaturated, hydrocarbon radicals having at least 7 carbonatoms, preferably 12 to 23 carbon atoms. These compounds may besubstituted with fluorine. The fluorine-substituted lubricants may berepresented by the following structural formulas: ##STR2## where m has avalue from 1 to 5. Preferred examples of the radiation curablelubricants include stearic acid methacrylate and acrylate, methacrylateand acrylate of strearyl alcohol, glycerine methacrylate and acrylate,glycol methacrylate and acrylate, silicone methacrylate and acrylate,vinyl stearate, vinyl myristate, etc.

The topcoat layer contact compound, anti-oxidant, and optionallylubricant may be formed on the surface of the ferromagnetic thin film byany desired techniques, for example, by diluting the components in asolvent and thinly applying the resulting solution onto theferromagnetic thin film, or by evaporating the components in air orinert gas or vacuum and directing the resulting vapor to theferromagnetic thin film surface.

Various procedures may be taken, for example, by mixing theradiation-curable compound, antioxidant, and lubricant together, andapplying the mixture followed by curing, or by applying and curing amixture of the radiation-curable compound and antioxidant, coating orevaporating the lubricant onto the mixture film to form a lubricantcoating or deposition. Application of these additives alone or inadmixture may be carried out using a solvent. The additives may beevaporated or gas phase deposited by vaporizing them in air or inert gasor vacuum and directing the vapor to the surface of an object to becoated therewith. Application of the additives by evaporation isadvantageous in producing a layer with a uniform or smooth surface,leading to improved output waveform.

The proportion of the radiation-curable compound, antioxidant, andlubricant used in the topcoat layer may be controlled such that theratio of the radiation-curable compound to antioxidant ranges from 10:90to 90:10, and preferably from 30:70 to 70:30 by weight. The lubricantmay be added in amounts of from 0.5 to 100 parts by weight per 100 partsby weight of the radiation-curable compound plus antioxidant. Smalleramounts of the radiation-curable compound outside this range result in atopcoat layer which is less tough and liable to abrasion. Amounts of theantioxidant outside this range are too small to provide rust preventiveeffect. Then the ferromagnetic metal thin film tends to be corrodedwhich in turn, causes damage to the topcoat layer, resulting indeteriorated electromagnetic properties as evidenced by an output drop.

The topcoat layer according to the present invention has many advantagesattributable to the respective components. That is, theradiation-curable compound reinforces the topcoat layer whichexperiences less abrasion; the anti-oxidant provides a sufficient rustpreventive effect; and the lubricant serves to reduce the frictionalresistance on the magnetic layer surface.

These effects provide magnetic recording media with moving stability,durability or runnability, and reduced dropout.

The anti-oxidant and the lubricant contained in the topcoat layer areboth preferably of radiation curable type.

Radiation

Active energy radiation used for crosslinking may include electronradiation from a radiation source in the form of a radiationaccelerator, γ-ray emitted from Co60, β-ray emitted from Sr90, X-rayemitted from an X-ray generator, and ultraviolet radiation. Particularlypreferred radiation for exposure is radiation generated by a radiationaccelerator because of simple control of radiation dose, incorporationin a manufacturing line and electromagnetic radiation shielding. Incuring the topcoat layer through exposure to radiation, it is preferredto operate a radiation accelerator at an accelerating voltage of 100 to750 kV, and preferably 150 to 300 kV to generate radiation having asufficient penetrating power such that the object is exposed to aradiation dose of 0.5 to 20 megarad. In the practice of radiation curingaccording to the present invention, it is very advantageous to use a lowdose type radiation accelerator (electrocurtain system) available fromEnergy Science Inc. of United States America because it may be readilyincorporated in a tape coating and fabricating line and internalshielding of secondary x-rays is complete. A van de Graaff typeaccelerator may equally be employed which have been widely used as aradiation accelerator in the prior art.

It is important in radiation crosslinking to expose the topcoat layer toradiation in a stream of an inert gas such as N₂ gas and He gas.Exposure to radiation in air is not desirable because O₃ generated byradiation exposure acts on the binder polymer to generate radicalstherein which in turn, adversely affect the crosslinking reaction of thebinder. It is thus important that the atmosphere where active energyradiation is irradiated be an atmosphere of an inert gas such as N₂, Heand CO₂ having an oxygen concentration of 5% at the maximum.

A photo polymerization sensitizer may be added to the topcoatcomposition according to the present invention with the advantage ofpromoted ultraviolet curing. The photo polymerization sensitizers usedherein may be selected from well-known sensitizers. Examples of suchsensitizers include benzoins such as benzoin methyl ether, benzoin ethylether, α-methylbenzoin, α-chlorodeoxybenzoin, etc.; ketones such asbenzophenone, acetophenone, bis(dialkylamino)benzophenones; quinonessuch as anthraquinone and phenanthraquinone; and sulfides such as benzylsulfide, tetramethylthiuram monosulfide, etc. The photo polymerizationsensitizers may be added in amounts of 0.1 to 10% by weight based on theresin solids.

Backcoat

The backcoat layer used in the practice of the present inventioncontains an inorganic pigment, an organic binder, and a lubricant.

The inorganic pigments used herein include

(1) electroconductive carbon black and graphite, and

(2) inorganic fillers such as SiO₂, TiO₂, Al₂ O₃, Cr₂ O₃, SiC, CaO,CaCO₃, zinc oxide, goethite, α-Fe₂ O₃, talc, kaolin, CaSO₄, boronnitride, fluorinated graphite, molybdenum disulfate, ZnS, etc., withCaCO₃, kaolin, ZnO, goethite, ZnS and carbon black being preferred.

The amount of the inorganic pigment used may be 20 to 200 parts byweight for type (1) and 10 to 300 parts by weight for type (2) per 100parts by weight of the binder. Backcoats containing higher contents ofthe inorganic pigment become brittle so that increased dropouts mayoccur.

The lubricants used in the backcoat layer along with dispersants usedtherefor may be selected from conventional ones commonly used inbackcoats of this type in the prior art. Examples of the lubricantsinclude fatty acids having at least 12 carbon atoms, moreillustratively, RCOOH where R is an alkyl having at least 11 carbonatoms, such as caprylic acid, capric acid, lauric acid, mirystic acid,palmitic acid, stearic acid, behenic acid, oleic acid, elaidic acid,linolic acid, linoleic acid, stearolic acid, etc.; metal soaps, forexample, soaps of the foregoing fatty acids with alkali metals such asLi, Na and K and alkaline earth metals such as Mg, Ca and Ba; andlecithin, etc. Also included are higher alcohols having at least 12carbon atoms, and sulfate esters thereof, surface active agents,titanium coupling agents, silane coupling agents, etc.

Additional examples of the lubricants include silicone oil, graphite,molybdenum disulfide, tungsten disulfide, and fatty acid esters ofmonobasic fatty acids having 12 to 6 carbon atoms with monohydricalcohols having 3 to 12 carbon atoms, and fatty acid esters of monobasicfatty acids having at least 17 carbon atoms with monohydric alcohols,the latter esters having 21 to 23 carbon atoms in total.

The lubricants (along with the associated dispersants) may be added inan amount of 0.2 to 20 parts by weight per 100 parts by weight of thebinder.

The backcoat layers may further contain any of well-known additivescommonly used in backcoats of this type in the prior art.

One class of such additives includes anti-static agents, for example,natural surface active agents such as saponin; nonionic surface activeagents such as alkylene oxide series, glycerine series, and glycidolseries agents; cationic surface active agents such as higher alkylamines, quaternary ammonium salts, pyridine and similar heterocycliccompounds, phosphonium salts, and sulfonium salts; anionic surfaceactive agents such as carboxylic acids, sulfonic acid, phosphoric acid,and those containing acidic radicals such as sulfate and phosphateresidues; amphoteric surface active agents such as amino acids,aminosulfonic acid, and sulfate and phosphate esters of amino-alcohols.

The binders used in the backcoat layers in the practice of the presentinvention may be selected from thermoplastic, thermosetting andreactive-type resins which have been commonly used in prior art magneticrecording media, and mixtures thereof. Among them, thermosetting resins,and especially radiation curable resins are preferred because of thestrength of the resultant coating.

The thermoplastic resins used herein are resins having a softening pointof lower than 150° C., an average molecular weight of 10,000 to 200,000,and a polymerization degree of about 200 to 2,000, for example, vinylchloride-vinyl acetate copolymers (which may have carboxylic unitsincorporated therein), vinyl chloride-vinyl acetate-vinyl alcoholcopolymers (which may have carboxylic units incorporated therein), vinylchloride-vinylidene chloride copolymers, vinyl chloride-acrylonitrilecopolymers, acrylate-acrylonitrile copolymers, acrylate-vinylidenechloride copolymers, acrylate-styrene copolymers,methacrylate-acrylonitrile copolymers, methacrylate-vinylidene chloridecopolymers, methacrylate-styrene copolymers, urethane elastomers,nylon-silicon resins, nitrocellulose-polyamide resins, polyfluorovinylresins, vinylidene chloride-acrylonitrile copolymers,butadiene-acrylonitrile copolymers, polyamide resins, polyvinyl butyral,cellulose derivatives (such as cellulose acetate, cellulose diacetate,cellulose triacetate, cellulose propionate, nitrocellulose, etc.),styrene-butadiene copolymers, polyester resins, chlorovinylether-acrylate copolymers, amino resins, various synthetic rubber seriesthermoplastic resins, and mixtures thereof.

The thermosetting and reactive-type resins are resins which have amolecular weight of less than 200,000 in solution form to be applied,and after being applied, dried, and heated, have an infinitely increasedmolecular weight as a result of condensation and addition reactions.Among them, preferred are those resins which do not soften or meltbefore they are pyrolyzed.

Illustrative of these resins are phenol resins, epoxy resins,polyurethane setting resins, urea resins, melamine resins, alkyd resins,silicone resins, acrylic resins, acrylic reactive resins,epoxy-polyamide resins, nitrocellulose melamine resins, mixtures of highmolecular weight polyester resins and isocyanate prepolymers, mixturesof methacrylate copolymers and diisocyanate prepolymers, mixtures ofpolyester polyols and polyisocyanates, urea-formaldehyde resins, lowmolecular weight glycol/high molecular weight diol/triphenylmethanetriisocyanate mixtures, polyamine resins, and mixtures thereof.

Among them particularly preferred are thermosetting resin compoundscomprising a cellulosic resin (pyroxylin etc.), a vinyl chloride-vinylacetate-vinyl alcohol copolymer, and urethane with a curing agent added.Also preferred are radiation-curable resin compounds comprising a vinylchloride-vinyl acetate-vinyl alcohol copolymer (which may havecarboxylic units incorporated therein) or acrylic-modified vinylchloride-vinyl acetate-vinyl alcohol copolymer (which may havecarboxylic units incorporated therein) and urethane acrylate.

In addition to the above preferred combinations, also preferred arethose thermoplastic resins having contained or incorporated in theirmolecule radicals susceptible to crosslinking or polymerization uponexposure to radiation, for example, acrylic double bonds as given byacrylic and methacrylic acids having an unsaturated double bond capableof radical polymerization or esters thereof; allyl double bonds as givenby diallyl phthalate; and unsaturated bonds as given by maleic acid andmaleic derivatives.

Monomers which can be used as the binder component in the practice ofthe present invention include acrylic acid, methacrylic acid, andacrylamide.

The preferred examples and their combination of radiation-curable binderin the backcoat layer are similar to those of the topcoat.

If a thermosetting organic binder is used in the manufacture of themagnetic recording medium of the present invention, the lubricant in thebackcoat can be transferred to the magnetic thin film duringmanufacturing process, resulting in many disadvantages includingunstable movement of the medium in service accompanied by outputreduction or even image signal loss, rather high friction, andseparation or breakage of the ferromagnetic thin film due to backwardmorphology transfer. These problems may be overcome by first applyingthe topcoat while there often occurs another problem that the medium astop coated is susceptible to damage during the process. Further, in thecase of thermosetting type, the back-to-top morphology transfer causedby tightening of a jumbo roll of coated medium during thermosettingprocess gives rise to a difference in electromagnetic properties betweenouter and inner coils of the roll.

On the contrary, radiation-curable resins can be continuously curedwithin a relatively short time during manufacturing process, thuscompletely eliminating the back-to-top morphology transfer and hence,minimizing dropouts. The steps of applying and curing the topcoat can beperformed on-line in the manufacturing process, contributing to costreduction through energy consumption and labor savings. Advantages inquality are also achieved by reducing the dropout in a magnetic tapewhich is otherwise remarkable due to tightening during curing, andeliminating an output difference of the magnetic tape in a lengthwisedirection due to the difference in stress between outer and inner coilsof a roll of tape.

The radiation-curable resin binder comprising components (A), (B), and(C) will be discussed in further detail. Component (A) alone isunflexible and brittle, component (B) alone is less elastic, andcombinations of (A) and (B) are sufficient to increase breaking energy,but insufficient to increase brittle energy, and become tacky toincrease static friction under high temperature, high humidityconditions probably because of low hardness.

Further combining component (C) with components (A) and (B) increasesthe crosslinking property of the binder sufficiently to form a fullycured tough backcoat film exhibiting a high tensile strength, breakingenergy, brittle energy, and abrasion resistance (being little abradedaway). It has been found that magnetic recording medium samplestop-coated with a binder composition of (A), (B), and (C) do not becometacky and remain low in friction coefficient and free of reproducedimage distortion even after storage in a high-temperature environment at50° C. and relative humidity 80% for 5 days. It is proved that thecrosslinking property and hence, curing degree of the backcoat film isincreased by the addition of (C). The addition of (C) to (A) and (B)allows component (A) having a lower molecular weight to be employed ascompared with component (A) available in the case of combinations of (A)and (B). The originally plastic component (A) is improved in plasticityand curing degree by introducing component (C) so that a highlyviscoelastic film having an increased brittle energy may be obtained.

The limitations on components (A), (B), and (C) constituting theradiation-curable resin binders will be briefly described. If (A) has amolecular weight of less than 5,000 and/or (B) has a molecular weight ofless than 3,500, there results a too hard backcoat film which will besubstantially abraded away and lead to deteriorated electromagneticproperties. If the molecular weight of (B) exceeds 100,000, thereresults poor dispersion and hence, deteriorated electromagneticproperties. Particularly when such (B) is of radiation curable type, itscuring property is adversely affected, resulting in reduced strength Ifthe molecular weight of (C) exceeds 3,000, crosslinking property isdeteriorated and film strength is lowered.

Preferably, component (A) has a molecular weight of 10,000 to 80,000,(B) has a molecular weight of 3,000 to 80,000, and (C) has a molecularweight of 200 to 2,500. The use of component (B) of radiation-curabletype is most preferred because of promoted crosslinking and increasedfilm strength.

Components (A), (B), and (C) are blended in such proportion that thecomposition contains 20 to 70%, and preferably 30 to 70% of (A), 20 to80%, and preferably 20 to 60% of (B), and 10 to 40%, and preferably 10to 30% of (C), all percents being by weight.

It should be noted that the molecular weight of components (A), (B), and(C) used herein designates a number average molecular weight asdetermined by gel permeation chromatography (G.P.C.). More particularly,the G.P.C. is a liquid chromatography process of the type using a columnfilled with porous gel serving as a molecular sieve for separatingmolecules in a sample in terms of their dimensions in a mobile phase.The average molecular weight may be determined by passing a polystyrenehaving a predetermined molecular weight as a reference sample to depicta calibration curve on the basis of elution time. Then the averagemolecular weight is calculated on the polystyrene basis. If a given highmolecular weight substance contains Ni molecules having a molecularweight Mi, then the number average molecular weight Mn is represented bythe equation: ##EQU1##

The components (A), (B), and (C) used in the backcoat binder accordingto the invention should contain per molecular at least 2, and preferablyat least 5 unsaturated double bonds for (A), at least 1, and preferablyat least 5 unsaturated double bonds for (B), and at least 1, andpreferably at least 3 unsaturated double bonds for (C).

Particularly preferred for the radiation curable resin composition usedin the backcoat according to the invention are combinations of

component (A) selected from partially saponified vinyl chloride-vinylacetate copolymers, vinyl chloride-vinyl acetate copolymers havingcarboxylic units incorporated therein, and compounds prepared byreacting isocyanate-bearing compounds resulting from reaction betweenphenoxy resins and polyisocyanate compounds with acrylic or methacryliccompounds having a functional group capable of reacting with theisocyanate group,

component (B) selected from compounds prepared by reacting isocyanatecompounds or polyols (polyurethane elastomers) resulting from reactionbetween polyols and isocyanates with acrylic or methacrylic compoundshaving a reactive functional group,

component (C) selected from polyfunctional acrylate or methacrylatemonomers, oligoester acrylates, and low molecular weight compounds of(B).

The organic binders, lubricants and anti-oxidants used in the backcoatlayer in the practice of the invention are preferably of radiationcurable type.

Active energy radiation used for crosslinking, radiation source andirradiation conditions are as previously mentioned.

A photo polymerization sensitizer as previously described may also beadded to the backcoat composition according to the present inventionwith the advantage of promoted ultraviolet curing.

In the fabrication of conventional magnetic recording media having athermosetting backcoat layer, the backcoat layer should be formed afterthe formation of the magnetic layer because a roll of a backcoatedsubstrate film experiences tightening of coils or tightened contact ofthe backcoat to the substrate face surface during heat treatment,undesirably reducing the surface roughness of the substrate facesurface. For this reason, the magnetic layer must be formed before theheat curing process. It is thus a common practice to apply a backcoatlayer to the back surface of the substrate film having a magnetic layeralready formed thereon. On the contrary, the use of a radiation curablebinder in the practice of the present invention allows either a topcoator a backcoat to be formed in advance because the radiation curablebinder can be cured immediately after application on a fabricating linewithout subsequent tightening in rolled form.

Magnetic Head

The magnetic recording medium of the present invention may be operatedin combination with a variety of magnetic heads. It is preferred that atleast a gap-defining edge portion of the magnetic head be of a magneticmetal material. It is possible to form a core entirely of aferromagnetic metal material although a part of the core including agap-defining edge portion may be formed of a ferromagnetic metalmaterial.

FIG. 2 schematically shows a magnetic head generally designated at 20 ascomprising core halves 21 and 22 formed of a ferromagnetic material suchas ferrite. The core halves 21 and 22 are metallized at theirgap-defining edge portions with ferromagnetic metal material layers 31and 32 of about 1 to 5 μm thick by sputtering or any suitablemetallizing techniques. The core halves 21 and 22 are mated so as todefine a gap 24 therebetween which is filled with glass or dielectricmaterial and has a distance a. This configuration, although the figureis not drawn to exact proportion and shape, provides improvedelectromagnetic properties and ensures smooth tape passage thereacrosswithout head adhesion or clogging. Of course, the shape and structure ofthe head is well known.

In the practice of the present invention, it is desirable that the headgap 24 has a distance a of 0.1 to 0.5 μm, and preferably 0.1 to 0.4 μm,and a track width of 10 to 50 μm, and preferably 10 to 20 μm.

The ferromagnetic metal materials used in the fabrication of themagnetic head may be selected from a variety of such materials includingthin films and thin plates of amorphous magnetic metals, Sendust, hardPermalloy, Permalloy, etc. Among them, particularly preferred areamorphous magnetic Co-based alloys because they experience little headadhesion or clogging and have excellent electromagnetic properties.Preferred are amorphous magnetic alloys comprising 70 to 95 atom % of Coand 5 to 20 atom % of a vitrifying element(s) such as Zr, Nb, Ta, Hf,rare earth elements, Si, B, P, C, Al, etc., with the Zr and/or Nb beingmost preferred. Also preferred are alloys comprising 65 to 85 atom % ofCo and 15 to 35 atom % of Si and/or B as a vitrifying element. Thelatter alloys may further contain less than 10 atom % of Fe, less than25 atom % of Ni, less than 20 atom % (in total) of at least one memberof Cr, Ti, Ru, W, Mo, Ti, Mn, etc. Less desirable results were obtainedwhen a Sendust head was used.

The amorphous magnetic alloys may be formed into core halves orgap-defining segments by sputtering or high speed quenching.

Recording/reproducing operation may be performed on the magneticrecording medium of the present invention by means of theabove-mentioned magnetic head in accordance with any well-known videorecording/reproducing systems including the so-called VHS, Beta, 8-mmvideo and U-standard systems.

The magnetic recording medium and recording/reproducing method accordingto the present invention has a number of benefits.

The magnetic recording medium exhibits sufficiently reduced dynamicfriction to provide stable movement.

Runnability is outstandingly improved so that the dynamic frictionincreases little after repeated travel cycles in a recording/reproducingequipment. The medium tolerates an increased number ofrecording/reproducing operations and offers improved stillcharacteristics (characteristics in the still mode reproduction).

Improved stability ensures that the medium can be stored and operated inseverely varying environments from high-temperature high-humidity tolow-temperature low-humidity environments.

Reproduction output is little affected by a spacing loss and containsless noise.

The magnetic recording medium operated in contact with a head releaseslittle materials which will adhere to and clog the head.

These benefits are further enhanced when the medium is provided with abackcoat and used in combination with ferromagnetic metal heads, andparticularly in the case of high density recording at a minimumrecording wavelength of less than 1 μm.

EXAMPLES

Examples of the present invention are given below by way of illustrationand not by way of limitation.

Example 1

Colloidal silica was applied onto a substantially particulate-freesmooth polyester film of 12 μm thick. There was obtained a substratehaving submicron particles or protrusions distributed thereon at adensity of 10⁷ /mm². The protrusions was as high as about 150 Å.

(1) Formation of magnetic layer

Ferromaonetic thin film 1

The substrate was moved along the circumferential surface of a cooledcylindrical can in a chamber which was evacuated to a vacuum of 1.0×10⁻⁴Torr. A 1:1 (by volume) mixture of O₂ and Ar was passed through thechamber at a flow rate of 800 cc/min. A 80/20 Co/Ni alloy was melted inthe chamber and evaporated toward the substrate within the range ofincident angle between 90° and 30° by the oblique evaporation technique.There was formed a Co-Ni-O thin film of 0.15 μm thick on the substrate.

Oxygen was locally concentrated at the interface with the substrate andthe surface of the magnetic film remote from the substrate. The surfaceof the magnetic layer remote from the substrate was substantiallycompletely covered with oxides. The magnetic film had a coercive forceHc=1,000 Oe. The average quantity of oxygen in the magnetic film was 40%as expressed by its atomic ratio to Co and Ni, that is, O/(Co+Ni)×100.

Ferromagnetic thin film 2

A ferromagnetic thin film 2 was prepared in the same manner as forferromagnetic thin film 1 except that the substrate was moved along thecircumferential surface of a cooled cylindrical can in a chamber whichwas evacuated to a vacuum of 5.0×10⁻⁶ Torr There was formed a thin filmconsisting essentially of Co-Ni and having a thickness of 0.15 μm.

The resulting tape was forcedly oxidized in an atmosphere at 9020 C. andRH 20% such that the surface of the magnetic film remote from thesubstrate consisted of oxides. The magnetic film had a coercive force Hcof 900 Oe. The average quantity of oxygen in the magnetic film was 45%as expressed by its atomic ratio to Co and Ni.

Ferromagnetic thin film 3 (comparison)

A ferromagnetic thin film 3 was prepared by the same evaporation processas for ferromagnetic thin film 1 except that the substrate was movedalong the circumferential surface of a cooled cylindrical can in achamber which was evacuated to a vacuum of 5.0×10⁻⁶ Torr. The oxidizingtreatment with oxygen as done for ferromagnetic thin film 2 was omitted.There was formed a thin film consisting essentially of Co-Ni and havinga thickness of 0.15 μm and a coercive force Hc=950 Oe.

(2) Formation of Backcoat

    ______________________________________                                        Backcoat layer 1         Parts by weight                                      ______________________________________                                        Zinc oxide, 80 μm     200                                                  Curing agent, Collonate L                                                                              20                                                   Lubricant, stearic acid-modified silicone                                                              4                                                    butyl stearate           2                                                    Nitrocellulose           40                                                   Vinyl chloride-vinyl acetate-vinyl alcohol                                                             30                                                   copolymer (Eslek A, Sekisui Chemicals K.K.)                                   Polyurethane elastomer (Essen 5703,                                                                    30                                                   B. F. Goodrich Co.)                                                           Mixed solvent, 1/1 MIBK/toluene                                                                        250                                                  ______________________________________                                    

Backcoat layer 1 was formed by applying the solution to the back surfaceof the polyester substrate having a thickness of 15 μm, evaporating offthe solvent with an infrared lamp or hot air, smoothening the surface,and heating the roll in an oven at 80° C. for 24 hours to promoteisocyanate crosslinking.

    ______________________________________                                        Backcoat layer 2        Parts by weight                                       ______________________________________                                        Carbon black, 100 μm (Asahi Carbon K.K.)                                                           50                                                    (A) Acryl-modified vinyl chloride-                                                                    50                                                    vinyl acetate-vinyl alcohol copolymer,                                        MW = 45,000                                                                   (B) Acryl-modified polyurethane elastomer,                                                            50                                                    MW = 5,000                                                                    Stearic acid             2                                                    Butyl stearate           2                                                    Mixed solvent, 1/1 MIBK/toluene                                                                       300                                                   ______________________________________                                    

Each mixture was dispersed in a ball mill for 5 hours and applied to theback surface of the polyester substrate having the magnetic layeralready formed thereon to a dry thickness of 1 μm. The resultingbackcoat layer was exposed to electronic radiation using an electroniccurtain type electron radiation accelerator at an accelerating voltage150 keV, electrode current 10 mA, and dose 5 Mrad in N₂ gas.

    ______________________________________                                        Backcoat layer 3       Parts by weight                                        ______________________________________                                        Zinc oxide, variable particle size                                                                   30                                                     Carbon black,          25                                                     (A) Acryl-modified vinyl chloride-                                                                   40                                                     vinyl acetate-vinyl alcohol copolymer,                                        MW = 30,000                                                                   (B) Acryl-modified polyurethane elastomer,                                                           40                                                     MW = 20,000                                                                   Polyfunctional acrylate, MW = 1,000                                                                  20                                                     Oleic acid             4                                                      Stearyl methacrylate   2                                                      Mixed solvent, 1/1 MIBK/toluene                                                                      250                                                    ______________________________________                                    

Backcoat layer 3 was prepared in the same manner as described forbackcoat layer 2.

    ______________________________________                                        Backcoat layer 4        Parts by weight                                       ______________________________________                                        CaCO.sub.3, 80 μm    50                                                    (A) Acryl-modified vinyl chloride-                                                                    30                                                    vinyl acetate-vinyl alcohol copolymer,                                        MW = 30,000                                                                   (B) Acryl-modified polyurethane elastomer,                                                            30                                                    MW = 50,000                                                                   Acryl-modified phenoxy resin, MW = 35,000                                                             20                                                    Polyfunctional acrylate, MW = 500                                                                     20                                                    Stearic acid            4                                                     Mixed solvent, 1/1 MIBK/toluene                                                                       300                                                   ______________________________________                                    

Backcoat layer 4 was prepared in the same manner as described forbackcoat layer 2.

(3) Formation of Topcoat Layer

    ______________________________________                                                              Parts by weight                                         ______________________________________                                        Topcoat composition 1                                                         2,6-di-tert-butyl-p-cresol,                                                                           1.6                                                   Acryl-modified vinyl chloride-                                                                        1                                                     vinyl acetate-vinyl alcohol copolymer                                         (having maleic units incorporated                                             therein), MW = 20,000                                                         MEK (methyl ethyl ketone)                                                                             100                                                   Topcoat composition 2                                                         Monoglycol salicylate acrylate                                                                        3                                                     Modified epoxy resin (having succinic                                                                 2                                                     units incorporated therein), MW = 6,000                                       Myristic acid           0.3                                                   Myristylalcohol methacrylate                                                                          1.0                                                   MEK/toluene 1/1         100                                                   Topcoat composition 3                                                         Dimethylamino methacrylate                                                                            1                                                     Modified phenoxy resin, MW = 30,000                                                                   0.6                                                   Acryl-modified polyurethane elastomer,                                                                0.6                                                   MW = 4,000                                                                    Fluorine (electrolytic process)                                                                       0.3                                                   MEK/toluene 1/1         100                                                   ______________________________________                                    

Preparation and Nature of Topcoat Layer

(1) Topcoat layer 1 was obtained by applying topcoat composition 1 ontothe ferromagnetic thin film followed by exposure to radiation at anaccelerating voltage of 150 keV and an electrode current of 6 mA to adose of 3 Mrad in N₂ gas. The topcoat layer was 20 Å thick.

(2) Topcoat layer 2 was obtained by applying topcoat composition 2 ontothe ferromagnetic thin film followed by exposure to radiation at anaccelerating voltage of 150 keV and an electrode current of 6 mA to adose of 3 Mrad in N₂ gas. The topcoat layer was 50 Å thick.

(3) Topcoat layer 3 was obtained by applying topcoat composition 3 ontothe ferromagnetic thin film followed by exposure to radiation at anaccelerating voltage of 150 keV and an electrode current of 10 mA to adose of 5 Mrad in N₂ gas. The topcoat layer was 40 Å thick.

    ______________________________________                                        Topcoat composition 4    Parts by weight                                      ______________________________________                                        (a)  Dimethylaminoethyl methacrylate                                                                       1                                                     Modified epoxy resin, MW = 80,000                                                                     0.5                                                   Acryl-modified polyurethane elastomer,                                                                0.5                                                   MW = 5,000                                                                    MEK/toluene 1/1         100                                              ______________________________________                                    

Composition (a) was applied onto the ferromagnetic thin film and thenexposed to radiation at an accelerating voltage of 150 keV and anelectrode current of 10 mA to a dose of 5 Mrad in N₂ gas.

    ______________________________________                                        (b)    Stearylmethacrylate   0.3                                                     Fluorine oil (telomerization process)                                                               0.2                                                     MEK                   100                                              ______________________________________                                    

Composition (b) was applied onto composition (a) and then exposed toradiation at an accelerating voltage of 150 keV and an electrode currentof 4 mA to a dose of 2 Mrad in N₂ gas The resulting topcoat layer was 50Å thick.

Topcoat composition 5

Film segment (a) of topcoat composition 4 on the ferromagnetic thin filmadsorbed stearyl alcohol in an atmosphere at 4×10⁻³ Torr. The resultingtopcoat layer was 30 Å thick.

Topcoat composition 6

Film segment (a) of topcoat composition 4 on the ferromagnetic thin filmadsorbed fluoride-modified oil in an atmosphere at 4×10⁻³ Torr. Theresulting topcoat layer was 20 Å thick.

Topcoat composition 7 (comparison)

Topcoat composition 7 is the same as topcoat composition 1 except thatthe radiation-curable compound is omitted and the antioxidant isincreased by an amount equal to that of the radiation-curable compoundomitted.

Topcoat composition 8 (comparison)

Topcoat composition 8 is the same as topcoat composition 3 except thatthe radiation-curable compound is omitted and the antioxidant isincreased by an amount equal to that of the radiation-curable compoundomitted.

Magnetic recording media were prepared using these ferromagnetic thinfilms and topcoat layers as shown in Table 1 along with theirproperties.

The magnetic head used in examining the media was of the type shown inFIG. 2 and having a gap distance a of 0.25 μm and a track width of 20μm. The core halves 21, 22 were formed of ferrite, gap-defining edgeportions 31, 32 were amorphous layers of Co 0.8/Ni 0.1/Zr 0.1 (atomicratio percent) formed by sputtering to a thickness of 3 μm, and the gapfiller was glass. For the head of this size, the minimum protrusiondistribution density 10⁵ /a² is calculated to be 1.6×10⁶.

A ferrite magnetic head of the same shape and size was also used forcomparison purpose.

The measurement of various properties is described below.

Still life

Signals are recorded in tape at 5 MHz and the reproduced output ismeasured for still life. A continuous operation in still mode in excessof 10 minutes is acceptable.

Friction on magnetic layer side

A tape is partially wound around a cylinder such that the magnetic layerside is in contact with the cylinder. While a load of 20 g is applied toone end of the tape, the cylinder is rotated 90°. A change of tension isdetermined for friction measurement.

Protrusion observation

Tape surface was observed under a scanning electron microscope (SEM) anda transmissive electron microscope (TEM).

Output

Signals are recorded and reproduced at a frequency of 5 MHz to determinean S/N ratio (relative value). To this end, a commercially available VHStype video tape recorder is modified so as to make possible measurementat 5 MHz.

Head clogging

A tape is moved 50 passes on a commercially available VHS type videotape recorder. It is observed how the head gap is clogged.

Head adhesion

A tape is moved predetermined passes on a commercially available VHStype video tape recorder. Then the head drum is removed and examined onthe tape contacting surface under an optical microscope.

Guide adhesion

A tape is moved predetermined passes on a commercially available VHStype video tape recorder. Then the guide pin is removed and examinedunder an optical microscope for the adhesion of tape material.

Topcoat wear

A tape is moved predetermined passes on a commercially available VHStype video tape recorder. Then the tape is observed under an opticalmicroscope.

                  TABLE 1                                                         ______________________________________                                              Ferromagnetic                                                                             Top     Back Head    Still                                  No.   thin film   coat    coat adhesion                                                                              life, min.                             ______________________________________                                        1     1           1       1    slight  >10                                    2     1           2       2    none    >10                                    3     1           3       3    none    >10                                    4     1           4       2    none    >10                                    5     1           5       3    slight  >10                                    6     1           6       4    none    >10                                    7     2           2       1    none    >10                                    8     2           4       3    slight  >10                                    9     2           6       4    slight  >10                                    10*   3           1       1    middle    1                                    11*   1           7       1    middle  >10                                    12*   1           8       2    middle  >10                                    13*   2           7       3    middle  >10                                    14*   2           8       4    middle  >10                                    ______________________________________                                              Coefficient of                                                                friction**   Guide**   Head**  Output**                                 No.   on magnetic side                                                                           Adhesion  Clogging                                                                              Stability                                ______________________________________                                        1     0.20         slight    none    good                                     2     0.20         none      none    good                                     3     0.19         none      none    good                                     4     0.20         slight    none    good                                     5     0.21         slight    none    good                                     6     0.19         slight    none    good                                     7     0.20         none      none    good                                     8     0.20         slight    none    good                                     9     0.20         slight    none    good                                     10*   top coat layer peeled off                                               11*   0.25         middle    severe  poor                                     12*   0.26         middle    severe  poor                                     13*   0.25         middle    severe  poor                                     14*   0.27         middle    severe  poor                                     ______________________________________                                         *comparative example                                                          **40° C., 60%, 50 passes                                          

EXAMPLE 2

Certain properties of magnetic recording tapes are shown in relation tothe height and density of protrusions on the magnetic layer surface.

Experiments were carried out using signals having a minimum recordingwavelength of 0.7 μm. The magnetic heads used were the same amorphoushead as used in Example 1 and a ferrite head of the same shape and size.

The topcoat layer used was topcoat layer 4 and the backcoat layer usedwas backcoat layer 3 both used in Example 1.

The magnetic layer was formed in the same conditions as used forferromagnetic thin film 1 in Example 1.

                  TABLE 2                                                         ______________________________________                                        Sam- Protrusion                                                               ple  Height, Density,/ Reproducing                                                                            Head                                          No.  Å   cm.sup.2  Output, dB                                                                             clogging                                                                             Head                                   ______________________________________                                        21    50     2 × 10.sup.8                                                                      +0.5     slight Amorphous                              22    50     5 × 10.sup.9                                                                      0        slight Amorphous                              23   100     4 × 10.sup.8                                                                      -0.1     none   Amorphous                              24   100     5 × 10.sup.9                                                                      -0.1     none   Amorphous                              25   100     .sup. 2 × 10.sup.10                                                               -0.3     none   Amorphous                              26   200     3 × 10.sup.8                                                                      -0.3     none   Amorphous                              27   200     4 × 10.sup.9                                                                      -0.4     none   Amorphous                              28   200     .sup. 3 × 10.sup.10                                                               -0.4     none   Amorphous                              29   300     2 × 10.sup.8                                                                      -0.3     none   Amorphous                              30   300     3 × 10.sup.9                                                                      -0.2     none   Amorphous                              31   300     .sup. 3 × 10.sup.10                                                               -0.5     none   Amorphous                              32   --      --        0        severe Amorphous                              33   1000    3 × 10.sup.8                                                                      -8.0     severe Amorphous                              34   100     1 × 10.sup.7                                                                      -0.1     slight Amorphous                              35   100     4 × 10.sup.8                                                                      -2.0     middle Ferrite                                36   200     4 × 10.sup.9                                                                      -2.2     middle Ferrite                                37   300     2 × 10.sup.8                                                                      -2.0     middle Ferrite                                38   1000    3 × 10.sup.8                                                                      -11      severe Ferrite                                ______________________________________                                    

All the ferromagnetic thin films in Examples were found by Augerspectroscopy to be covered with an oxide layer of 100 to 200 Å thick ontheir surface.

Although the foregoing Examples used colloidal silica as the inorganicsubmicron particles, equivalent results are obtained by using othersubmicron particles such as alumina sol, titanium black, zirconia, andvarious hydrosols.

In addition, equivalent results were obtained when a magnetic headformed of an amorphous Co-Fe-Ru-Cr-Si-B alloy was used. Less desirableresults were obtained when a Sendust head was used.

What is claimed is:
 1. A magnetic recording medium comprising:a flexiblesubstrate having opposed major surfaces, a ferromagnetic metal thin filmlayer on one surface of the substrate primarily comprising cobalt, anorganic topcoat layer on the surface of the metal thin film layer, and abackboat layer on the surface of the substrate, the magnetic recordingmedium being used in combination with a magnetic head having a gap,characterized in that said organic topcoat layer contains aradiation-curable compound and an anti-oxidant, said backboat layercontains an inorganic pigment, an organic binder, and a lubricant, saidmetal thin film layer contains oxygen, and the medium has protrusions onits surface at an average density of at least 10⁵ /a² per squaremillimeter of the surface, where a is the distance of the magnetic headgap as expressed in μm and is from 0.1 to 0.4 μm, said protrusionscorresponding to the submicron particles of a size of 30 to 300 Å withinthe medium, said protrusions having a height of 30 to 300 Å.
 2. Amagnetic recording medium according to claim 1 wherein said organictopcoat layer further contains a lubricant.
 3. A magnetic recordingmedium according to claim 1 wherein said flexible substrate is comprisedof a high polymer, and the protrusions conform to submicron particleshaving a size of 30 to 300 Å which are placed on one surface of thesubstrate.
 4. A magnetic recording medium according to claim 1 whereinsaid ferromagnetic metal thin film layer at the surface is covered witha ferromagnetic metal oxide coating.
 5. A magnetic recording mediumaccording to claim 1 wherein the organic binder in the backcoat layer isa radiation curable resin.
 6. A magnetic recording medium according toclaim 1 wherein the organic binder in the backcoat layer is acomposition comprising(A) a plastic compound having at least tworadiation-curable unsaturated double bonds and a molecular weight of5,000 to 100,000, (B) a rubber-like compound having at least oneradiation-curable unsaturated double bond or not being radiation curableand having a molecular weight of 3,000 to 100,000, and (C) a compoundhaving at least one radiation-curable unsaturated double bond and amolecular weight of 200 to 3,000.
 7. A magnetic recording mediumaccording to claim 1 wherein the anti-oxidant in the organic topcoatlayer is comprised of a radiation curable anti-oxidant.
 8. A magneticrecording medium according to claim 2 wherein the lubricant in theorganic topcoat is comprised of a radiation curable lubricant.
 9. Amagnetic recording medium according to claim 1 wherein said organictopcoat layer has a thickness of 10 to 100 Å.
 10. A method forconducting recording/reproducing operation on a magnetic recordingmedium comprising a flexible substrate having opposed major surfaces, aferromagnetic metal thin film layer on one surface of the substrateprincipally comprising cobalt, an organic topcoat layer on the surfaceof the metal thin film layer, and a backcoat layer on the other surfaceof the substrate, by passing the medium across a magnetic head having agap, characterized in thatsaid organic topcoat layer contains aradiation curable compound an anti-oxidant, said backcoat layer containsan inorganic pigment, an organic binder, and a lubricant, said metalthin film layer contains oxygen, and the medium has in average at least10⁵ /a² protrusions per square millimeter of the surface, where a is thedistance of the magnetic head gas as expressed in μm and being from 0.1to 0.4 μm, said protrusions corresponding to submicron particles of asize of 30 to 300 Å within the medium, and said protrusions having aheight of 30 to 300 Å.
 11. A magnetic recording/reproducing methodaccording to claim 10 wherein the organic topcoat layer further containsa lubricant.
 12. A magnetic recording/reproducing method accord to claim10 wherein at least an edge portion of the magnetic head including thegap is formed of a ferromagnetic metal material.
 13. A magneticrecording/reproducing method according to claim 10 wherein theferromagnetic metal material is a magnetic amorphous cobalt base alloy.